Before I knew any cheats I actually used the fact that enemies spawned at the edge of the screen to my advantage. I picked up the machinegun, got to a part where an enemy just kept spawning onto the screen, and then taped the button down to keep firing at the edge of the screen so I could get a bunch of free lives from all the kills while I went to the pool.

When I did learn a cheat, it wasn't what is now called the Konami code, but the Up Up Down Down Left Right Left Right B A B A Select Start.